1. Field of the Invention
The present invention relates to a coherent light source that includes a semiconductor laser and a wavelength converting device and is employed, e.g., in the fields of optical information processing and optical measurement and to a recording/reproducing apparatus using the coherent light source.
2. Description of the Related Art
A small short-wavelength light source is necessary to achieve high-density optical disks and high-accuracy optical measurement. In particular, a recording/reproducing system using a hologram has drawn attention as a next-generation high-density optical disk because a recording density in the 100 Gbit/inch2 range can be expected.
FIG. 12 shows the schematic configuration of a conventional holographic optical information recording/reproducing system. This is, e.g., an optical system for optical disks that employs shift-multiplex recording proposed by Psaltis et al.
As shown in FIG. 12, a laser beam emitted from a laser light source 46 passes through a beam expander 47, where the beam diameter is expanded, and is divided by a half mirror 48. Consequently, the beam deflected in one direction passes through a spatial light modulator (hereinafter, referred to as “SLM”) 49 and is focused on a hologram disk 51 by a Fourier transform lens 50 so as to become a signal beam. The beam deflected in the other direction is converted through an aperture 52 into a beam having an appropriate diameter that acts as a reference beam, with which the same position of the signal beam on the hologram disk 51 is irradiated. The hologram disk 51 includes two glass substrates and a hologram medium, such as a photopolymer, sealed between the glass substrates. The interference fringes generated by the signal beam and the reference beam are recorded on the hologram disk 51.
The SLM 49 includes a two-dimensional array of light switches, each of which is turned on and off separately in response to the input signal to be recorded. For example, the SLM having 1024×1024 cells can display information of 1 Mbit at the same time. The information of 1 Mbit displayed on the SLM 49 by the signal beam passing through it is converted into a two-dimensional array of light beams, which then is recorded on the hologram disk 51 as interference fringes. For reproduction of the recorded signal, the hologram disk 51 is irradiated with the reference beam alone, and the diffracted light from the hologram is received by a CCD device 53.
The holographic optical recording system uses a hologram medium having a large thickness of about 1 mm and records the interference fringes as a thick grating, i.e., a so-called Bragg grating, thereby enabling angular-multiplex recording. The system in FIG. 12 can achieve angular multiplexing by shifting the position irradiated with the reference beam of spherical wave instead of changing the angle of incidence of the reference beam. Specifically, the multiplex recording is performed in such a manner that the hologram disk 51 is rotated slightly so as to shift the recorded position, causing a small change in the angle of incidence of the reference beam detected by each portion of the hologram medium.
The angular selectivity that depends on the intensity of a reproduced signal for a hologram medium having a thickness of 1 mm is as follows: the full-width at half-maximum is 0.014 degrees; the holograms are multiplexed at intervals of about 20 μm when NA for the reference beam is 0.5. This makes it possible to achieve a recording density of 200 Gbit/inch2, which is 300 GB in terms of a 12 cm-disk capacity.
Since the Bragg grating has angular selectivity and wavelength selectivity, the wavelength of a light source during recording and reproducing has to be controlled. For the hologram medium having a thickness of 1 mm, the wavelength selectivity of the grating is 0.24 nm.
To achieve a high-density optical information recording/reproducing system as described above, a small stable laser light source and a recording medium for multiplex recording are important technologies. Generally, a solid laser (e.g., a YAG laser) or a gas laser (e.g., an Ar laser) is used as a laser light source because an absolute value of the oscillation wavelength is stable.
As a small short-wavelength light source, a coherent light source has drawn attention. The coherent light source includes a semiconductor laser and an optical waveguide-type second harmonic generation (hereinafter, referred to as “SHG”) device employing quasi-phase-matching (hereinafter, referred to as “QPM”), i.e., an optical waveguide-type QPM-SHG device (see Yamamoto et al., Optics Letters Vol. 16, No. 15, 1156 (1991)).
FIG. 13 shows the schematic configuration of an SHG blue light source including an optical waveguide-type QPM-SHG device. As shown in FIG. 13, a wavelength-variable distributed Bragg reflection (hereinafter, referred to as “DBR”) semiconductor laser 54 having a DBR region is used as a semiconductor laser. The semiconductor laser 54 is a 0.85 μm-band 100 mW-class AlGaAs wavelength-variable DBR semiconductor laser and includes an active region 56, a phase control region 57, and a DBR region 58. The oscillation wavelength of the semiconductor laser 54 can be varied continuously by changing the current to be input to the phase control region 57 and the DBR region 58 simultaneously.
An optical waveguide-type QPM-SHG device 55 used as a wavelength converting device includes an X-cut MgO-doped LiNbO3 substrate 59, an optical waveguide 60, and a periodic polarization inversion region 61. The optical waveguide 60 and the periodic polarization inversion region 61 are formed on the substrate 59. The optical waveguide 60 is produced by proton exchange in pyrophosphoric acid. The periodic polarization inversion region 61 is produced by forming comb-shaped electrodes on the substrate 59 and applying an electric field to those electrodes.
In the SHG blue light source shown in FIG. 13, when the laser output is 100 mW, a laser beam of 60 mW is coupled to the optical waveguide 60. The oscillation wavelength of the semiconductor laser 54 is fixed within the phase-matching wavelength tolerance of the QPM-SHG device 55 (i.e., the wavelength converting device) by controlling the amount of current to be input to the phase control region 57 and the DBR region 58 of the semiconductor laser 54. This SHG blue light source provides blue light of about 10 mW having a wavelength of 425 nm. The blue light has diffraction limited focusing properties when its transverse mode is a TE00 mode and reduced noise performance, i.e., −140 dB/Hz or less in relative noise field intensity.
As described above, in a holographic optical recording system, the diffraction patterns to be recorded are changed with the incidence direction and wavelength of light. Therefore, when the wavelength of light for recording is different from that for reproduction, cross-talk signals are increased and the signal beam intensity is degraded.
Information stored on the hologram disk 51 shown in FIG. 12 is reproduced as Bragg diffracted light from the interference fringes recorded. To reproduce the information with a sufficient quantity of light, it is necessary to meet the Bragg conditions. Specifically, the angle of incidence of a reference beam with respect to the hologram medium and the wavelength thereof each have to be adjusted to an optimum value.
For example, assuming that the system includes a hologram medium with a thickness of 1 mm, a light source with a wavelength of 515 nm and interference fringes with a period of 0.5 μm, the wavelength tolerance of a reference beam under the Bragg conditions is 515 nm±0.24 nm, which is defined by a wavelength at which the diffraction efficiency is reduced by half.
Moreover, thermal expansion of the hologram medium has to be taken into consideration because it causes a change in the period of the interference fringes recorded and a variation in the optimum wavelength of the reproducing light that meets the Bragg conditions.
The following is an explanation of an example where OmniDex 352, i.e., a photopolymer manufactured by Dupont, is used as a hologram medium. The linear thermal expansion coefficient of the hologram medium is 7.1×10−5 (see JP 5(1993)-16538 A). The amount of variation in the optimum wavelength over the range of temperature change of 25° C. is 0.18%, which is 515+0.9 nm in terms of the oscillation wavelength of an Ar laser. This value is more than three times the wavelength tolerance of 515±0.24 nm that meets the Bragg conditions. To achieve stable hologram reproduction over the range of temperature change of the hologram medium, it is necessary to control the wavelength of a light source for reproduction optimally according to a change in temperature of the hologram medium during reproducing.